This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
The present disclosure relates generally to methods and systems for performing a wavefield separation of sonic data using cross-correlation. In particular, the present disclosure relates to methods and systems to obtain event signals of reflected and transmitted waves of sonic data in oil and gas industries.
Event signals are extracted from observed waveforms of sonic data by estimating and subtracting direct phases. In general, the direct phases in a waveform trace are predicted by examining signals in adjacent traces that form a common moveout curve in a set of waveform traces (waveform trace gather). There are selections of grouping and sorting methods of waveform traces for a wavefield separation as same as for the conventional processing of sonic data. The common offset gather (COG) is a set of traces whose source and receiver distance is constant. In this gather, Hornby (1989) applied a f-k filter to remove the direct phases and unwanted reflected waves (such as reflected Stoneley waves) as described in the document of Hornby, B. E., 1989, “Imaging near-borehole of formation structure using full-waveform sonic data”: Geophysics, 54, pp. 747-757. Li et al. (2002) applied a median filter after correcting P-waves arrivals as described in the document of Li, Y., R. Zhou, X. M. Tang, J. C. Jackson, and D. Patterson, 2002, “Single-well imaging with acoustic reflection survey” at Mounds, Okla., USA: 64th Conference & Exhibition, EAGE, Paper P141. The common shot gather (CSG, receiver section) is a set of traces of a common source location and multiple receiver locations. Tang (1997) separated direct phases using the parametric estimation of direct phases (for example Lang et al. (1987)) as described in the documents of Tang, X. M., 1997, “Predictive processing of array acoustic waveform data”: Geophysics, 62, pp. 1710-1714, and Lang, S. W., A. L. Kurkjian, J. H. McClellan, C. F. Morris, and T. W. Parks, 1987, “Estimating slowness dispersion from arrays of sonic logging waveforms”: Geophysics, 52, pp. 530-544. Haldorsen et al. (2005) applied the adaptive interference canceller (AIC) filter as described in the document of Haldorsen, J., W. Borland, H. A. B. Al Rougha, A. Sultan, and R. Meehan, 2005, “Azimuthal sonic imaging”: 67th Conference & Exhibition, EAGE, Paper 1-017. The common receiver θgather (CRG, source section) of Hsu and Chang (1987) and Tang (1997) is a set of traces of a common receiver location and multiple shot locations as described in the document of Hsu, K., and S. K. Chang, 1987, “Multiple-shot processing of array sonic waveforms”: Geo-physics, 52, 1376-1390, and the document of Tang (1997). As the directions of moveout curves of reflected event signals in CSG and COG are opposite whereas those of direct phases are common, the event signals can be effectively extracted by switching these gathers so that the directions of moveout curves of event signals become opposite to those of direct phases (Tang (1997)). Note that the contents of the foregoing documents of Hornby (1989), Li et al. (2002), Tang (1997), Lang et al. (1987), Haldorsen et al. (2005) and Hsu and Chang (1987) are incorporated herein in their entirety by reference thereto.
There is a need, however, for improving the accuracy of estimation of direct phases to obtain the event signals.